This invention relates generally to length-adjustable shafts and more particularly to quick-change positive locking telescoping shafts.
There are innumerable applications in many diverse fields for length-adjustable shafts with positive locking features. One such application is in telescoping adjustable steering columns of motor vehicles, in which it is desirable to be able to quickly change the length of the steering column and to in positively lock it in the selected position.
Many length-adjustable shafts have quick-change capability based on frictional locking mechanisms, similar to collets, in which a tapered ring is threaded on a split tube member to frictionally lock a mating tube or shaft in position within the split tube. These are quite effective, but they provide clamping force which is limited by the operator""s skill in tightening the threaded ring and also by wear and contamination of the clamping surfaces. Also, they do not provide positive locking since the clamping force can be overcome by a large axial load on the shaft.
Many other current shafts have rack and pawl adjustment locks in which a pawl is pivoted away from a rack on the sliding member of the shaft to allow change of length. When released, the pawl pivots back into engagement with the rack by spring action to lock the shaft in position.
Currently, still other such shafts are made by threading the inside of a tube and the outside of a mating shaft and then axially cutting away slightly more than half the threads on both members by machining flats on opposing sides of the shaft and arced grooves on opposing sides of the inside of the tube. The shaft can then be inserted in the tube with its remaining threads aligned with the arced grooves of the tube so it slides into the desired position. The shaft is given a quarter turn to lock it in any position relative to the tube. To readjust the length of the shaft/tube couple, it is only necessary to turn either member one-quarter turn and slide it to the new position and re-lock it. These provide quick adjustment and are capable of positive locking, but the presence of burrs at the edges of the machined flats and grooves often prevents turning the tube and shaft to the locked position. Even without burrs, the flats at the edges of the threads on the shaft and tube may butt against each other and prevent turning to the locked position; because they have no normal lead in as would be found in common threads.
The foregoing illustrates limitations known to exist in present quick-change adjustable length shafts. Thus, it would clearly be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.
In one aspect of the present invention, this is accomplished by providing a telescopic shaft for rapid length adjustment, comprising a tube having a plurality of substantially circumferential alternating ridges and grooves on at least a portion of an inner surface and having an oval cross-section with a major diameter DA and a minor diameter DB; an inner shaft member disposed within said tube and having matching ridges and grooves, the shaft also having an oval cross-section with a major diameter DC and a minor diameter DD; and the ridges and grooves of said tube interengaging with the grooves and ridges of said shaft when the shaft major diameter DC is substantially parallel with the tube minor diameter DB.
The foregoing and other aspects will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.